OP ARTS AND SCIENCES: APRIL 9, 1867. 



275 



With the exception of the substitutions i" and e", which will be con- 

 sidered hereafter, all those which are used in the reduction of the 

 moods of either oblique figure have the form of inferences in the same 

 figure. 



The so-called reductio per impossihile is the repetition or inversion 

 of that contraposition of propositions by which the indirect figures 

 have been obtained. Now, contradiction arises from a difference both 

 in quantity and quality ; but it is to be observed that, in the contrapo- 

 sition which gives the second figure, a change of the quality alone, and 

 in that which gives the thii'd figure a change of the quantity alone, of 

 the contraposed propositions, is sufficient. This shows that the two 



